Polish President Andrzej Duda has blocked the appointment of new generals, in a fresh sign of tension between the head of state and the ruling nationalist Law and Justice (PiS) party that originally backed him.

Duda cited problems with a proposed new command system under government reform plans as his reason on Tuesday for blocking the appointments. His move came two weeks after he unexpectedly vetoed two controversial bills intended to reform the judicial system.

A source close to Duda's administration said the nominations concerned around a dozen generals.

Deputy Defense Minister Tomasz Szatkowski said his ministry had invited the National Security Bureau, which advises the president, to help work on the army reform proposals.

"We are always available to the Bureau for additional clarifications or consultations," Szatkowski told Reuters.

But some security analysts said Duda - who won the presidency in 2015 thanks to the support of PiS - was keen to demonstrate again his independence as both president and as commander in chief of Poland's armed forces.

"This is probably less about the generals and more about the fact that the president decided it would be appropriate to say loudly and publicly that all options for a dialogue with the Defense Ministry have been exhausted," said Marek Swierczynski, a security analyst at Polityka Insight, a think-tank.

NATO frontline

The spat comes as Poland plays host to more NATO troops as part of efforts to bolster the alliance's eastern flank in the face of a more assertive Russia. Moscow has seized Ukraine's Crimea region and backs armed separatists in eastern Ukraine.

The Polish Defense Ministry presented a new defense concept in May that envisages significantly beefing up its armed forces and overhauling their command system. The president and the National Security Bureau have yet to respond to those plans.

Halina Szymanska, Duda's chief of staff, told the state TVP Info news channel there was no conflict between the president and the Defense Ministry, but added: “First the work on building the new command and control structures needs to be completed and only then personnel decisions will be taken.”

Tomasz Siemoniak, a former defense minister in the Civic Platform government that lost power to PiS in 2015, said Duda's stance was a challenge to the authority of PiS leader Jaroslaw Kaczynski, the most powerful man in Poland.

"The PiS leader is facing a difficult decision as to which side to support because this confrontation [between the president and Defense Ministry] is disastrous to the ruling camp and will be devastating for the military," he told Reuters.

About a quarter of the Polish top brass has quit since PiS took power, citing disagreements with Defense Minister Antoni Macierewicz over personnel and other decisions.

Swierczynski put the current vacancy rate among generals at nearly 50 percent, and he added that Duda's decision to make his disagreement with the Defense Ministry public could harm Poland's image in NATO as a stable, reliable ally.

“If we show that we cannot reach a compromise on such a key issue as national security ... what does that say?”